All the tips to getting a straight line are great, but the two BEST tips are:
-Glosscote
-Straight edge, & cut the tape in strips with a knife
What I do, is glosscote (or future floor shine, depending on my mood) the whole shell, allow it to dry for 24 hours. I cut the masking tape (normal blue painters tape, the 2" roll) into about 1/2" strips, using a straight edge to keep the lines perfect. Place the tape on the model starting from the edge of the new color and work away. Work slowly pressing the tape on with a burnishing tool, don't secure the opposite end to the model, the tape does not stretch. Once masked completely, glosscote again, and again allow 24 hours, THEN paint the new color. I tend to thin mine a bit more then normal, and use more air for the first 2 cotes, then a little less air on the 3rd, and usually final cote.
I'll try to video tape my progress on the C6044AC I'm working on.